[julia-users] Re: @inbounds is broken?

[vavasis]

Jacob,

Here's another gotcha: It's ok to put @inbounds before if but not elseif. 
 For example, this works:

function sqrtfirst{T}(a::Array{T, 1})
    @assert(size(a,1) >= 1)
    @inbounds if a[1] > 0
        @inbounds p = sqrt(a[1])
    elseif a[1] <= 0
        @inbounds p = sqrt(-a[1])
    end
    p
end

but this gives an error

function sqrtfirst{T}(a::Array{T, 1})
    @assert(size(a,1) >= 1)
    @inbounds if a[1] > 0
        @inbounds p = sqrt(a[1])
    @inbounds elseif a[1] <= 0
        @inbounds p = sqrt(-a[1])
    end
    p
end

I am a reasonably experienced programmer in Matlab and C++, but I am having 
a tough time figuring out this syntax. I would say that a language feature 
this difficult to understand should be deprecated and replaced with 
something simpler.

-- Steve Vavasis



On Monday, August 4, 2014 1:45:21 PM UTC-4, vav...@uwaterloo.ca wrote:
>
> Dear Julia users,
>
> The usage of the @inbounds macro is not explained the manual, and its 
> syntax appears to be strange.  Consider the three functions at the end of 
> this posting.  Only the third one works -- why?
>
> In general, I think @inbounds is broken.  Besides the weird syntax, it has 
> two other issues.  First, there is no way to apply the macro to one 
> subscript operation but not another in a long expression (as far as I 
> know).  Second, it is not extensible in the sense that if programmer A 
> implements his/her own array-like structure with his/her own getindex and 
> setindex operations, he/she might like to have two versions of 
> getindex/setindex, one safe/slower and the other unsafe/faster, but there 
> is no way for programmer A to detect whether user B, a user of his/her new 
> array-like structure, has requested @inbounds or not.
>
> I would like to propose the following replacement for @inbounds, which 
> solves all three problems.  Instead of a macro, there should be two 
> different subscript operations, say a[1] and a[$ 1 $], where the first is 
> safe/slow and the second is unsafe/fast.  The compiler will compile the 
> first as getindex/setindex and the second as getindexUnsafe/setindexUnsafe.
>
> -- Steve Vavasis
>  
>
>
> function sqrtfirst{T}(a::Array{T, 1})
>     @assert(size(a,1) >= 1)
>     @inbounds sqrt(a[1])
> end
>
> function sqrtfirst{T}(a::Array{T, 1})
>     @assert(size(a,1) >= 1)
>     return @inbounds sqrt(a[1])
> end
>
> function sqrtfirst{T}(a::Array{T, 1})
>     @assert(size(a,1) >= 1)
>     @inbounds return sqrt(a[1])
> end
>
>
>
>